Embodiments according to the invention relate to an apparatus and a method for processing an audio signal to focus an acoustic signal by an arrangement of a plurality of loudspeakers, wherein the acoustic signal is based on the audio signal.
Some embodiments according to the invention relate to an improvement of sound focusing by using psychoacoustic effects.
In some applications, a directed emission of sound is desired. In this connection, the sound energy emitted by the sound source is to propagate in a preferred direction only. One possible application may be a sound system, that intends to provide a sound from the stage only to a certain audience area in the auditorium. The remaining auditorium should not be affected and/or unnecessary sound reflections on room walls are to be avoided this way. In terms of energy, the directed emission of sound may provide the possibility to emit the sound energy only in the direction in which it is needed.
The way in which sound is emitted from a sound source depends on the ratio of sizes between the sound-emitting surface and the considered wavelengths. In the case of wavelength (λ) being considerably larger than the membrane diameter, for example a canonical membrane, a non-directed sound emission takes place (see “Zollner, M.; Zwicker, E.: Elektroakustik, Springer-Verlag Berlin Heidelberg New York, 3. Auflage, 1. korrigierter Nachdruck 1998”). If the ratio is inverted, an increasing directed sound emission takes place with rising frequency and decreasing wavelength.
For loudspeaker arrays, the size of the array may, at least, correspond to half the wavelength of the lowest frequency in order to be able to emit sound in a directed way by the loudspeaker array, for example. Therefore, very large arrays are necessitated in particular for focusing down to low frequencies.
For example, there are two approaches for realization. The basis of the first approach is that the emitting area is made as large as possible with respect to the longest wavelength to be emitted. This approach is used, for example, in the Line-Array-Technology (see “Urban, M.; Heil, C.; Baumann, P.: Wavefront Sculpture Technology, presented at the 11th AES-Convention, 2001 Sep. 21-24, New York”) used for large scale acoustic irradiation. By lining up acoustically-coupled single emitters, a large emitting membrane area is formed. In this approach, it is problematic that the dimensions of the sound source necessarily becomes unmanageably large.
If such large dimensions are not desired, a directed sound emission may be successful by decreasing the wavelength, instead of the size of the sound-emitting area, so that the ratio between the wavelength and the emitter size is met.
This approach is realized, for example, in ultrasonic loudspeakers (see EP 1 484 944 A2 or DE 699 21 558 T2). The problems of this approach consist in the non-proven harmlessness of the high ultrasonic doses for humans and in little low-frequency reproduction. Therefore, this approach is hardly used despite having been known for a longer period of time.
A possibility for extending the perceived low-frequency reproduction of sound sources is a use of a pyschoacoustic effect. It is known that the low frequency region perceived by humans may be enlarged by using pyschoacoustic effects. The reproduction bandwidth perceived by humans is not necessarily equal with the physically reproduced bandwidth of a sound source. By using pyschoacoustic effects, the reproduced signal may be changed such that a listener gets the impression that, for example, the perceived low-end cut off frequency is lower than the physically existing one.
This is done by processing the useful signal in such a way that the harmonic overtones of the fundamental waves are formed such that an enhanced low frequency impression develops. In this connection, the actual fundamental frequency only needs to be reproduced very weak or even not at all. An often-used pyschoacoustic effect is, for example, the missing fundamental effect. Here, the harmonic overtone structure of the signal is influenced such that despite of non-reproduced fundamental frequencies, the human believes to perceive these (see U.S. Pat. No. 6,134,330 or “Larsen, E.; Aarts, R. M.: Audio Bandwidth Extension, John Wiley & Sons, Ltd., West Sussex, England, 2004”).
Some further examples for the psychoacoustic effect are shown in “Be-Tzur, D. et al.: The Effect of MaxxBass Pyschoacoustic Bass Enhancement on Loudspeaker Design, 106th AES Convention, Munich, Germany, 1999”, in “Woon S. Gan, Sen. M. Kuo, Chee W. Toh: Virtual bass for home entertainment, multimedia pc, game station and portable audio systems, IEEE Transactions on Consumer Electronics, Vol. 47, No. 4, November 2001, page 787-794”, at “http://www.srslabs.com/partners/aetech/trubass_theory.asp”, at “http://vst-plugins.homemusician.net/instruments/virtual_bass_vb1.html”, at “http://mp3.deepsound.net/plugins_dynamique.php”, and at “http://www.srs-store.com/store-plugins/mall/pdf/WOW%20XT%Plug-inmanual.pdf”.
Further examples for sound focusing are shown in “DEGA-Empfehlungen 101, Deutsche Gesellschaft für Akustik e.V., März 2006”, in “Yoomi Hur, Seong-woo Kim, Young-cheol Park, Dae Hee Youn: Highly focused sound beamforming algorithm using loudspeaker array system, presented at the 125th AES-Convention, 2008 Oct. 2-5, San Francisco”, and in “Jung-Woo Choi, Youngtae Kim, Sangchul Ko, Jungho Kim: Super-directly loudspeaker array for the generation of personal sound zone, presented at the 125th AES-Convention, 2008 Oct. 2-5, San Francisco”.